[time-nuts] I've been thinking about a GPS receiver experiment

Magnus Danielson magnus at rubidium.se
Wed Oct 25 17:26:11 EDT 2017


Hei Ole Petter,

You don't want to look at the PPS in that case.
You want to look at how the receivers clock solution pans out.
Since the receivers clock is set but then ticks to the speed of the 
rubidium, you now got a high resolution frequency and phase comparison.

Depending on the clock-mode, you might or might not experience 
clock-reset events. Typically 1 ms shifts of clock time. One needs to 
handle that in case you are not able to drive the clock quick enough 
towards on average be where the GPS/UTC average solution drives it to.

Anyway, so there is a different clock message, just don't recall it from 
the top of my head. I just recall that the Novatels is fairly generous 
with this data

Cheers,
Magnus

On 10/25/2017 06:14 AM, Ole Petter Ronningen wrote:
> I did log the #TIME message for several weeks on an OEMV-3 a while back. 
> The results were a bit suspicious, so I checked with Novatel support - 
> turns out the PPS on the OEMV (and I presume that also holds for OEM4) 
> is derived from L1 only - and the jitter is nothing to brag about. So 
> for disciplining with PPS, something like a UBlox would be better as far 
> as I can tell.
> 
> The other option is to log the #RANGE-message from the Novatel, convert 
> to RINEX and solve with PPP, and use the output of that to adjust the 
> rubidium. The added benefit is that you'll have an excellent log of what 
> your reference is doing if you get odd results in some measurements.
> 
> Ole
> 
> On Wed, Oct 25, 2017 at 12:56 AM, Magnus Danielson 
> <magnus at rubidium.dyndns.org <mailto:magnus at rubidium.dyndns.org>> wrote:
> 
>     Skip,
> 
>     I would rather use the rich Novatel reports and read out the time
>     error and use that as your phase detector, then the normal PI-loop
>     stuff with an optional low-pass to add and then use that to steer
>     the rubidium.
> 
>     It's one of those, when I get time, projects.
> 
>     Cheers,
>     Magnus
> 
> 
>     On 10/25/2017 12:17 AM, Skip Withrow wrote:
> 
>         Hello time-nuts,
> 
>         I've been thinking about a GPS receiver experiment and just
>         wondering
>         if there are any opinions or prior experience that might save me
>         a lot
>         of time.
> 
>         What I have been thinking about doing is taking a GPS receiver
>         (Novatel OEM4-G2) that has provisions for an external clock (5 or 10
>         MHz) and driving it with a rubidium oscillator (that has 1pps
>         disciplining, (such as the X72 v5.05 or SRS PRS-10).  The GPS
>         even has
>         settings for OCXO/rubidium/cesium dynamics.
> 
>         Then, (and here is the unknown part) what if the GPS receiver
>         1pps is
>         used to discipline the rubidium?  This basically forms a feedback
>         loop, so could either hurt or help - depending.  Supposedly the
>         better
>         oscillator would give a better GPS solution.  And the better
>         solution
>         (1pps) should provide a better oscillator frequency.
> 
>         We know that GPS receivers using asynchronous clocks have 1pps
>         errors
>         and hanging bridges (OEM4 is spec'd at 20ns rms), If the
>         oscillator is
>         on 10MHz and disciplined will the 1pps error be reduced such as the
>         Thunderbolt?
> 
>         Comments appreciated.
> 
>         Regards,
>         Skip Withrow
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